From about 800 AD, the waves of Viking assaults on the coastlines
of the British Isles were gradually followed by a succession of Viking
settlers, bringing with them a culture and a tradition markedly different
from that of the existing Anglo-Saxon society. These enclaves
rapidly expanded and soon the Viking warriors were establishing areas
of control to an extent that they might reasonably be described as kingdoms.

In 878, after Alfred the Great of Anglia defeated the Viking
King Guthrum at the Battle of Edington, an area East of the old road from
London to Chester was left under the kingship of the Vikings (Danes and
Norsemen). Thus, in exchange for peace, the Danelaw -- the teritory
still occupied by Vikings -- was agreed upon by the English King.